1. Field of the Invention
The present invention relates to a method of discriminating an image, by which an image is discriminated on the basis of the data which is the image information obtained by photoelectrically scanning the image, and to a method of copying the image, and more particularly it relates to a method of discriminating an image, which, e.g., in copying an original, is based on finding the original's characteristics as the to what conditions should be selected for copying the original, and to a method of copying the image, in which the copying conditions of a copying apparatus is determined in accordance with the method of discriminating an image.
2. Description of the Prior Art
In copying apparatus or other equivalents, control systems for controlling copy densities according to the original's densities have already been devised and are now made practical reality. The image discriminating method used in such systems includes those which control the density condition according to the maximum value or the minimum value of the original's densities and those which control the density condition according to the mean value of the original's densities. In these methods, however, since it is difficult to make accurate image discrimination, unsatisfactory density control has been carried out.
On the other hand, an attempt has been made to form a density histogram with the density values from sampled pieces of image information to thereby perform image discrimination {for example, as disclosed in Japanese patent publication Open to Public Inspection (hereinafter referred to as Japanese Patent O.P.I. Publication) No. 45564/1982}. This method has an advantage over the above-mentioned methods in respect of the improvement of the image discrimination accuracy. However, this method also has a problem which is such that the discrimination accuracy is insufficient, or even if sufficient the procedure of the discrimination is troublesome.
Also, for the copying system wherein the copying conditions are determined by discriminating the original image, the following control algorithm has been devised: That is, an original image is in advance scanned to form a density histogram, from which the minimum density is found. The minimum density is then used to thereby determine the development bias voltage; provided, however, if the minimum density value is less than a given value, the development bias voltage is to be determined according to the density range, the width of the density histogram.
The copying method of the above control algorithm, however, has the disadvantage that, since the background density is detected from the minimum density level, if the original to be copied is one such as a blue print whose background is uneven in its density, the resulting copy image tends to produce a background fog. The method has an additional problem that, in the case of a low-density color background line-drawing original, even if the background fog is removed, the lines are reproduced with their density remaining low, so that the image becomes indistinct or illegible, and by contrast with this, if the image is reproduced in trying to make its contrast greater the background fog becomes conspicuous. The above method has a further problem that, if a gradational image, particularly one having a lot of high-density portions (black-and-white or color image) is copied, the copied image tends to become a solid-black or dark image because it takes no account of the image gradation.